Romii was the name of a planet, in the Romii star system, under Romulan control. (TOS episode: "Balance of Terror",TLE novel: The Art of the Impossible)

During the Dominion War, the A.C. Walden Medicine Show played a concert on Romii, which was attended by then-Commander Donatra, as well as Professor Sonek Pran. (ST novel: A Singular Destiny)

According to historians, Romii was one a large, single planetoid within Romii system. Around five-point three billion years ago, another planetary body which was sixty percent the size of Romii that had a retrograde orbit, collided with the planet. The resulting impact stripped the outer crust from Romii and in less then two hundred years, the ejecta from that planet would coalesce into two separate planets namely Romulus and Remus which would continue to spin with the force of that ancient, catastrophic collision. (ST novel: Captain's Blood)